Use of Molecular Scaffolding for the Stabilization of an Intramolecular Dative P^III-P^V System

The reaction of Napp(2)S(4) (1; Nap = naphthdlene-1,8-diyl) with chlorine gas gave [Nap(PCl2)(PCl4)] (2), displaying a rare sigma(4)P-sigma(6)P bonding interaction. An X-ray structure analysis confirmed the PCl5-like, P-P bond containing phosphonium-phosphoride structure of 2 in the crystal, which was also found in solution at low temperature. At ambient and higher temperatures, dynamic behaviour on the NMR time-scale was observed, which was assigned to interchange of the ionic phosphonium-phosphoride form 2 and the molecular bis(phosphorane) Nap(PCl3)(2) form 3, rather than to the ionic phosphonium salt-phosphorane form [Nap(PCl3)(PCl2)][Cl] 4. Electronic structure calculations were performed at the B3LYP/6-31G(d,p) level of theory on structures 2 and 3; structure 3 was located as a local minimum on the potential energy surface, 15 kcal.mol(-1) higher in energy than structure 2. The crystal structure and calculated P-P distances are 2.34 and 2.31 Angstrom for 2 and 3, respectively. An activation energy of 19.7 kcal.mol(-1) was found for the transition state structure by coordinate driving calculations; the line-shape analysis of variable temperature P-31{H-1} NMR spectra gave an activation energy of 14.4 kcal.mol(-1). (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.